Connected: An Internet Encyclopedia
RFC 950 - 2. A Class B Network Case

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### 2. A Class B Network Case

For this case, assume that the requesting host is on class B network 128.99.0.0, has address 128.99.4.123, that there is a gateway at 128.99.4.62, and that a 6-bit wide subnet field is in use, that is, the address mask is 255.255.252.0.

The host sends the ICMP request to 255.255.255.255:

             Source address:          128.99.4.123
             Destination address:     255.255.255.255
             Protocol:                ICMP = 1
             Type:                    Address Mask Request = AM1
             Code:                    0
             Mask:                    0

The gateway can then respond directly to the requesting host.

             Source address:          128.99.4.62
             Destination address:     128.99.4.123
             Protocol:                ICMP = 1
             Type:                    Address Mask Reply = AM2
             Code:                    0
             Mask:                    255.255.252.0

In the diskless workstation case the host sends:

             Source address:          0.0.0.0
             Destination address:     255.255.255.255
             Protocol:                ICMP = 1
             Type:                    Address Mask Request = AM1
             Code:                    0
             Mask:                    0

128.99.4.62 will hear the datagram, and should respond with this datagram:

             Source address:          128.99.4.62
             Destination address:     255.255.255.255
             Protocol:                ICMP = 1
             Type:                    Address Mask Reply = AM2
             Code:                    0
             Mask:                    255.255.252.0

If broadcasting is not allowed 128.99.4.123 sends:

             Source address:          128.99.4.123
             Destination address:     128.99.4.62
             Protocol:                ICMP = 1
             Type:                    Address Mask Request = AM1
             Code:                    0
             Mask:                    0

128.99.4.62 should respond exactly as in the previous case.

             Source address:          128.99.4.62
             Destination address:     128.99.4.123
             Protocol:                ICMP = 1
             Type:                    Address Mask Reply = AM2
             Code:                    0
             Mask:                    255.255.252.0

Next: 3. A Class C Network Case (illustrating non-contiguous subnet bits)

Connected: An Internet Encyclopedia
RFC 950 - 2. A Class B Network Case